390 



THE PERIPHERAL NERVOUS SYSTEM 



gins to ossify it separates from Meckel's cartilage. The incus is early connected 

 with the anlage of the stapes, and the connected portion becomes the crus longum. 

 Between this and the stapes an articulation develops. 



The stapes and Reichert's cartilage are derived from the second branchial 

 arch (Fig. 387). The mesenchymal anlage of the stapes is perforated by the 

 stapedial artery, and its cartilaginous anlage is ring-shaped. This form persists 

 until the middle of the third month, when it assumes its adult structure and the 

 stapedial artery disappears. 



Fig. 388. — Stages in the development of the auricle. (Adapted in part after His). ^,11 mm.; B, 

 13.6 mm.; C, 15 mm.; D, adult. 1, 2, 3, elevations on the mandibular arch; 4, 5, 6, elevations on the 

 hyoid arch; af, auricular fold; ov, otic vesicle; 1, tragus; 2, 3, helix; 4, 5, antihelix; 6, antitragus. 



The muscle of the malleus, the tensor lympani, is derived from the first branchial arch; 

 the stapedial muscte from the second arch. The further fact that these muscles are innervated 

 by the trigeminal and facial nerves, which are the nerves of the first and second arches re- 

 spectively, points toward a similar origin for the ear ossicles. 



Fuchs, studying rabbit embryos, on the contrary, concludes: (1) the stapes is derived 

 from the capsule of the labyrinth; (2) the malleus and incus arise independently of the first 

 branchial arch. 



The External Ear. — The external ear is developed from and about the first 

 ectodermal branchial groove. The auricle arises from six elevations which appear, 

 three on the mandibular, and three on the hyoid arch (Fig. 388). Modern ac- 



